NRAO Press Release

Research Teams Will Use Very Large Array
To Study Close-Passing Comet

Four teams of scientists are preparing to use the National Science
Foundation's (NSF) Very Large Array (VLA) radio telescope to
study a bright comet making the closest approach to Earth of any
comet in more than a decade. On March 25, Comet Hyakutake, already
a naked-eye object in the morning sky, will pass within 10 million
miles of Earth. Discovered on Jan. 30 by Japanese amateur
astronomer Yuji Hyakutake, this comet is exciting scientists who
expect to learn a great deal from studying it.

Soon after its discovery, astronomers suspected that Hyakutake
might prove to be a very bright comet. Around the world,
professional and amateur astronomers prepared to make the most of
the opportunity.

"Much of the progress in cometary research comes from the study of
the relatively bright comets," said Patrick Palmer of the
University of Chicago. Though several comets a year approach
close enough to be studied, observations of "typical" comets
often are difficult to interpret, Palmer said, because the
observations are sparse and the comets relatively faint. "Bright
comets solve these problems," he said, because their brightness
produces data of high quality and the bright comets "excite
interest so that more research groups make observations."

Researchers will use the NSF's VLA for three principal types of
studies. Three of the groups will seek to detect thermal radio
emission from the comet. One of these groups also will look for
the characteristic radio emission of particular molecules in the
comet's coma. A fourth group will use the VLA as a receiver for a
powerful radar signal sent from Goldstone, CA, bounced off the
comet and returned to Earth. In four days near the comet's closest
approach to Earth, more than 36 hours of VLA observing time will
be devoted to its study.

Thermally-generated radio emission from the comet will,
scientists hope, provide valuable information. Thermal radio
emission has been detected from the nucleus of a comet only once
before. One team, led by Carey Lisse of the University of
Maryland, hopes that variations in the radio brightness of this
emission from the nucleus may allow determination of its rotation
period.

Wilhelm Altenhoff, from Germany, and Bryan Butler, of NRAO in
Socorro, will study thermal emission from Hyakutake's coma,
trying to learn about the sizes of the particles making up the
coma and of the coma itself.

In addition, Palmer says, studying the comet's thermal radio
emissions helps scientists understand "the heat balance and
indirectly, the composition, of comets."

Comet Hyakutake also will be observed at very specific radio
wavelengths that indicate the presence of particular molecules,
including formaldehyde, methanol and ammonia.

Comets, "dirty snowballs" of ices and dust, are believed to be
made of material left over from the formation of the planets and
moons of the solar system. As such, they offer scientists an
opportunity to learn about the presolar nebula from which the
solar system was formed. Radio observations are an important part
of this research, Palmer said, because many of the molecules
scientists seek to find in comets "are most easily seen and
identified at radio wavelengths."

"Detection of these molecules is very important to enable us to
answer the question of what comets are made of. More than 100 of
these molecules have been identified in the spectrum of
interstellar gas in star-forming regions. So far, less than 20
have been seen in comets. Is this because of the intrinsic
difficulty of observing comets or because something different has
happened to the gas that was incorporated into comets when the
solar system was formed?" Palmer said.

A research team headed by Imke de Pater of the University of
California at Berkeley will use the VLA as the receiving portion
of a radar system that will bounce microwave signals off the
comet. The California transmitter for this system is operated by
NASA's Jet Propulsion Laboratory. Using this radar system, the
scientists hope to learn details about a halo of centimeter-sized
particles around the nucleus of the comet. Such a halo "has never
been directly detected before," de Pater said, but has been
inferred.

The radar observations, Palmer added, will help "measure the
distribution of centimeter-sized particles around comets." These
are the particles that, when their orbital paths cross that of
Earth, are seen as bright meteors. Palmer said the scientists hope
to learn what proportion of these particles are large enough to
survive their entry into the atmosphere and actually hit the
Earth's surface. "This is an important question when considering
the source of the meteors that do hit Earth," he said.

Many of the VLA observations are in conjunction with observations
by other telescopes, both radio and optical, around the world.
Astronomers are using a wide variety of ground-based and orbiting
telescopes to gain the most possible information about the comet.
"No telescope other than the VLA is capable of carrying out either
the radar experiment or the detection of long-wave thermal
emission," said Lisse.

The VLA is an instrument of the National Radio Astronomy
Observatory (NRAO). The NRAO is a facility of the National Science
Foundation, operated under cooperative agreement by Associated
Universities, Inc.

Hyakutake's approach to within 10 million miles of Earth is the
closest pass by a comet since 1983, and the fifth closest pass for
any comet this century. The close approach of Hyakutake, Lisse
said, "will allow us to make measurements that we cannot make for
most comets." The comet's proximity also helps the radar
observation, said de Pater, since "the closer the object, the
stronger is the radar echo." The record for a close approach by a
comet is held by Comet Lexell, which passed within 1.4 million
miles of Earth in 1770.

The comet, which Hyakutake discovered using a pair of binoculars
with lenses six inches across, will make its closest approach to
the sun on May 1. According to Brian Marsden of the International
Astronomical Union's Central Bureau for Astronomical Telegrams,
Comet Hyakutake (officially Comet C/1996 B2 (Hyakutake)) is in an
orbit which brings it close to the sun once every 10,000 or 20,000
years.

Thousands of amateur astronomers already have viewed the comet,
which can be seen with the naked eye in areas relatively free of
excessive outdoor lighting. Across the country, astronomy clubs,
planetaria and observatories will be sponsoring comet-viewing
sessions for the public.